This invention relates to a light measuring device for a camera with automatic exposure control, which device measures the light reflected by the leading shutter member of a focal plane shutter and/or at the film plane to effect exposure control according to the measured light output obtained.
A known camera light measuring optical system is such that a light measuring element and convex lens, serving as a condensing element, are disposed on the bottom wall of the camera casing whereby light reflected by the upper and lower portions of the surface of a leading shutter member as well as at the film plane are collected through a convex lens onto the light measuring element.
The light measuring optical system of the type described tends to lose its balanced light measurement, in case the convex lens, namely a condensing element, is positioned too close to the focal plane of the objective lens. This is due to the fact that there arises a marked difference in the length of the optical path between: (a) the light reflected by upper portions of the surface of the leading shutter member as well as at the film plane and incident on the light measuring element; and (b) the light reflected by lower portions of the surface of the leading shutter member and the film plane, and incident on the light measuring element. Consequently, the intensity of reflected light by the upper portions of the surface of the leading shutter member is much lower than the light reflected by the lower portions. Thus, it follows that only the light from the lower portions of the surface of the leading shutter member and/or the film plane are measured. Stated otherwise, where the camera is held sideways, an optimum exposure cannot be achieved for a major lower portion of the object if a bright light portion, such as sky, lies in an upper part of the object. Furthermore, there remains some distance between the plane of the leading shutter member and the film plane disposed behind that shutter member, so that there is a vertical difference in the zones of the object which are to be light-measured before and after the travelling of the shutter curtain. If well-balanced light measurement is not obtained as set forth in the foregoing, when it is desired to take a picture of an object having a bright light portion, such as sky, in an upper portion thereof, such vertical difference in the zones of the object results in an increased difference in the light-measured output before and after movement of the shutter curtain.
To eliminate the above-described drawback, it is desirable, in the light measuring optical system of the type described, to mount the condensing element as far towards the front of the camera casing from the focal plane as possible. From the standpoint of the size of the camera casing, it is impossible to mount the condensing element too far towards the front of the camera casing with respect to the focal plane. In prior art light measuring optical systems using a convex lens as a condensing element, the light measuring element is spaced further apart from the focal plane than the convex lens, so that well-balanced light measurement is not obtained unless the camera casing is enlarged.